Indicia reading terminals and decoding imagers for reading and decoding decodable indicia are available in multiple varieties. For example, minimally featured indicia reading terminals devoid of a keyboard and display are common in point of sale applications. Indicia reading terminals devoid of a keyboard and display are available in the recognizable gun style form factor having a handle and trigger button (trigger) that can be actuated by an index finger. Indicia reading terminals having keyboards and displays are also available. Keyboard and display equipped indicia reading terminals are commonly used in shipping and warehouse applications, and are available in form factors incorporating a display and keyboard. In a keyboard and display equipped indicia reading terminal, a trigger button for actuating the output of decoded messages is typically provided in such locations as to enable actuation by a thumb of an operator. Indicia reading terminals in a form devoid of a keyboard and display or in a keyboard and display equipped form are commonly used in a variety of data collection applications including point of sale applications, shipping applications, warehousing applications, security check point applications, and patient care applications. Some indicia reading terminals are adapted to read bar code symbols including one or more of one dimensional (1D) bar codes, stacked 1D bar codes, and two dimensional (2D) bar codes. Other indicia reading terminals are adapted to use optical character recognition (OCR) to read standard characters while still other indicia reading terminals are equipped to read both bar code symbols and OCR characters. Digital devices with imaging subsystems, such as smartphones, tablet computers, and other formats of mobile computers, may also be used for capturing and performing attempted decodes on image frames having one or more decodable features, such as characters, words, sentences, one dimensional (1D) barcodes, stacked 1D barcodes, and two dimensional (2D) barcodes, in any of a variety of formats, for example.
Some indicia reading terminals and decoding imagers employ charge-coupled device (CCD) based image sensors. A CCD based image sensor contains an array of electrically coupled light sensitive photodiodes that convert incident light energy into packets of electric charge. In operation, the charge packets are shifted out of the CCD imager sensor for subsequent processing.
Some indicia reading terminals and decoding imagers employ complementary metal-oxide semiconductor (CMOS) based image sensors as an alternative imaging technology. As with CCDs, CMOS based image sensors contain arrays of light sensitive photodiodes that convert incident light energy into electric charge. Unlike CCDs, however, CMOS based image sensors allow each pixel in a two-dimensional array to be directly addressed. One advantage of this is that sub-regions of a full frame of image data can be independently accessed, for a cropped or windowed frame of image data. Another advantage of CMOS based image sensors is that in general they have lower costs per pixel. This is primarily due to the fact that CMOS image sensors are made with standard CMOS processes in high volume wafer fabrication facilities that produce common integrated circuits such as microprocessors and the like. In addition to lower cost, the common fabrication process means that a CMOS pixel array can be integrated on a single circuit with other standard electronic devices such as clock drivers, digital logic, analog/digital converters and the like. This in turn has the further advantage of reducing space requirements and lowering power usage.
CMOS based image readers have traditionally employed rolling shutters to expose pixels in the sensor array. In a rolling shutter architecture, rows of pixels are activated and read-out in sequence. The exposure or integration time for a pixel is the time between a pixel being reset and its value being read-out. The exposure periods for adjacent rows of pixels typically overlap substantially as, in a typical example, several hundred rows of pixels must be exposed and read during the capture of a frame of data. The rolling shutter architecture with its overlapping exposure periods requires that the illumination source remain on during substantially all of the time required to capture a frame of data so that illumination is provided for all of the rows. In operation, the rolling shutter architecture also suffers from at least two imaging disadvantages: image distortion and image blur. Image distortion is an artifact of the different times at which each row of pixels is exposed. The effect of image distortion is most pronounced when fast moving objects are visually recorded, where different parts of the object are imaged while the object is at different positions, so that the object is distorted in the image. Image blur is an artifact of the long exposure periods typically required in a rolling shutter architecture in an image reader. As indicated above, in a rolling shutter architecture the illumination source must remain on during substantially all of the time required to capture a frame of data. Due to battery and/or illumination source limitations, the light provided during the capture of an entire frame of data is usually not adequate for short exposure times. Without a short exposure time, blur inducing effects become pronounced. Common examples of blur inducing effects include the displacement of an image sensor due to, for example, the unsteadiness of a user's hand with a hand held image reader.
As the pixel density of imagers used in barcode scanners and other imaging devices increases, a major trade-off against resolution is the loss in sensitivity due to a reduced pixel size. Typical VGA CMOS imager pixel size may be five to six microns, compared to a megapixel imager which could have pixels as small as 1.1 microns. As a result, maintaining sensitivity as resolution goes up becomes a very big challenge.
Typical CMOS imager pixels each have a photosensitive region and an opaque shielded data storage region that temporarily stores the imaging data from that pixel before readout and does not serve to absorb light, and therefore reduces the pixel's fill factor and its sensitivity. Typical CMOS image sensors also have crosstalk between pixels, that may be in the range of ten to twenty percent.
Uniformly illuminating a scene with high intensity illumination is always a challenge in typical barcode scanning applications. Very bright illumination is required to achieve a high contrast image at short exposure times needed for adequate motion tolerance. This may involve using multiple light sources, which adds to the size of a scanner and imposes requirements for higher currents, resulting in greater power demand and thermal implications. Multiple illumination sources and higher power demand imposes design challenges on a small integrated barcode scanner.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.